This is my 1st time to try a linux desktop on Ubuntu 14.04 with a hope to have a stable and usable version of linux.
It worked well at the beginning with some popular applications at the first day.

However, for unknown reason, (maybe after installing Dropbox or after a system update), the software center was disappeared and all software installed via software center were gone together.

System Settings cannnot be launched. Then I tried this command: sudo apt-get install --reinstall gnome-control-center

So system settings can be opened again but most items in system setting are not displaying.
There are only 4 items there including Printer, Security and Privacy, Language, and Software & Update. All others are not there.

When I click About this computer, instead of displaying computer info, it displays System Settings now.

So what's going on with this? It's so frustrated.

BTW, I am an experienced Windows user and couldn't understand why the core components are disappeared in Ubuntu. Can you image that Control Panel is disappeared in Windows? In Windows, I can try system restore or safe mode. It seems I don't have options to try in Ubuntu.